1. Field of the Invention
The present invention relates to the field of measuring and manufacturing of optical elements. In particular, the invention relates to a method of measuring and manufacturing an optical element having opposite first and second surfaces which are substantially flat surfaces oriented substantially parallel to each other or substantially spherical surfaces having substantially coinciding centers of curvature.
2. Brief Description of Related Art
The optical element may comprise, for example, an optical component such as a lens used in optical systems, such as telescopes used in astronomy and systems used for imaging structures, such as structures formed on a mask reticle, onto a radiation sensitive substrate, such as a resist, in a lithographic method. The success of such optical system is substantially determined by an accuracy with which the optical element can be machined or processed to have a target shape determined by a designer of the optical system. In such manufacture it is necessary to compare a shape of the processed optical surface with its target shape, and to determine differences between the processed and target surfaces. The optical element may then be further processed at those portions where differences between the machined and target surfaces exceed e.g. predefined thresholds.
In order to precisely measure the shapes of the optical surfaces of the optical element, an interferometer apparatus is commonly used. An example of such interferometer apparatus is a Fizeau interferometer. The conventional Fizeau interferometer usually includes a reference surface which is also referred to as a Fizeau surface and which is illuminated with measuring light. A portion of the measuring light is reflected back from the Fizeau surface and directed onto a camera. Another portion of the measuring light traverses the Fizeau surface and is incident on the optical surface under test to be reflected therefrom such that it is also incident on the camera. The light reflected from the Fizeau surface and the light reflected from the reference surface generate an interference pattern on the camera. By analysing this pattern, shape differences between the optical surface and its target shape can be determined in terms of wavelengths of the measuring light at respective locations on the optical surface.
If the optical element has two surfaces which are parallel to each other or which have coinciding centers of curvatures, measuring light reflected from both surfaces will be incident on the detector to interfere with the measuring light reflected from the Fizeau surface. Interference patterns generated from three interfering beams are difficult to analyze with a sufficient precision to determine shape errors of the respective optical surface to a desired accuracy. From US 2004/0190002 A1 there is known a method of suppressing a contribution of a selected optical surface of such optical element to the generated interference pattern by using measuring light of alternating wavelengths. However, this conventional method is complicated due to a necessity of a particular control of a wavelength tunable light source. Further, a thickness of the optical element may not always be determined with a desired accuracy by using the conventional methods.